1. Field of the Invention
This invention relates to a process of detecting the position of electric discharge generated between a discharge electrode of an electric discharge machine and a work and also to a process of electric discharge machining which permits electric discharge machining to be continued in a satisfactory state according to detected position information obtained by the electric discharge position detection process.
2. Description of the Prior Art
A discharge electrode of an electric discharge machine usually has a certain extent. For example, the discharge electrode of a wire electric discharge machine extends linearly (i.e., one-dimensionally), while the discharge electrode of a die sinking electric discharge machine extends plane-wise (i.e., two- or three-dimensionally). In the electric discharge machine, the machining is continued by generating electric discharge between the discharge electrode and the work. While the machining is being done, it is impossible to control the exact position in the discharge electrode having the certain extent where electric discharge is generated. However, the electric discharge generation position in the discharge electrode is closely related to the result of machining. For example, where electric discharge generation positions are concentrated in the discharge electrode, satisfactory result of machining can not be obtained. Where machining is made with electric discharge generation positions distributed uniformly, satisfactory result of machining can be obtained. It is thus desired to be able to detect the position of electric discharge in the discharge electrode generated during machining.
Concerning this purpose, a report entitled "Observation of Electric Discharge Points Distribution in Electric Discharge Machining" is provided in Electric Machining Technique (Journal of the Society of Electric Machining Engineers of Japan) Vol. 15, No. 49 (1991), pp. 13-22. The report proposes a technique of detecting electric discharge generation position in the following way. A figure in the report is annexed as FIGS. 11(A) and 11(B) to the present specification.
(1) A power supply line X for supplying discharge current i0 is branched into two or more branch power supply lines X1, X2, . . . which are connected at different points Y1, Y2, . . . to a discharge electrode Z. PA1 (2) With this arrangement, the discharge current i0 that is supplied through the line X is branched into two or more branch power supply lines X1, X2, . . . , and the discharge current divisions i1, i2, . . . are supplied from the two or more points Y1, Y2, . . . through the two or more branch power supply lines X1, X2, . . . to the discharge electrode Z. PA1 (3) The discharge current divisions i1, i2, . . . flowing through the branch power supply lines X1, X2, . . . are detected upon reaching of the steady state (i.e., at timings of T2 to T3, T6 to T7, and T10 to T11 in FIG. 11(B)). PA1 (4) The discharge current divisions i1, i2, . . . are each standardized after an equation (i1-i2)/(i1+i2). The standardized value that is obtained in this way, is zero when i1&gt;i2, and has a positive value when i1 i2 and has a negative value when i1&lt;i2.
(5) As is obvious from the equivalent circuit shown in FIG. 11(A), i1=i2 takes place when r0+rc+R1=r0+rc+R2, that is, R1=R2, where r0 is the resistance of each of the branch power supply lines X1 and X2, rc is the contact resistance of each branch power supply line with respect to the discharge electrode Z, R1 is the resistance of the discharge electrode Z between the connection point Y1 and the electric discharge generation position S, and R2 is the resistance of the discharge electrode Z between the connection point Y2 and the electric discharge generation position S. Obviously, with electric discharge generation at the mid point between the connection points Y1 and Y2, R1=R2 is obtained, and the standardized value is zero (see timings T9 to T12). When the connection point Y1 is approached by the electric discharge generation position S, we have i1&gt;i2 (see timings T1 to T4), and the standardized value is positive. If the value is large, the electric discharge generation position S is close to the connection point Y1. When the connection point Y2 is approached by the electric discharge generation position S, we have i1&lt;i2 (see timings T5 to T8), and the standardized value is negative. The greater the absolute value, the closer the electric discharge generation position S is to the connection point Y2. FIG. 12(A) shows the relation between the standardized value and the electric discharge generation position S. The position S is shown as a distance from the connection point Y1.
(6) In this technique, the electric discharge generation position S is detected according to the standardized value based on the theory described in (5). More specifically, the standardized value is obtained, and the electric discharge generation position is calculated from this value and the relation shown in FIG. 12(A). For removing the influence of induced electromotive force, the discharge current detection in the step in (3) is made after the discharge current divisions i1, i2, . . . have reached substantially the steady state.
In the commonly termed coarse machining, a large discharge current pulse length is taken, and a substantially steady state is obtained by the discharge current divisions in every time of pulse energization. Thus, in the prior art, it is possible to obtain electric discharge generation position detection.
However, when the pulse length is reduced for satisfactory machining, the pulse energization in every time may be ended before reaching of the steady state, thus making it impossible to detect the discharge current divisions i1, i2, . . . without the influence of induced electromotive force. Besides, the necessity for the electric discharge generation position detection is particularly high in a finish machining stage with reduced pulse length. Therefore, in the prior art process, the electric discharge generation position can not be detected when it is most desired to know the position.
As is obvious from the above description in (5), in the prior art, the electric discharge generation position detection is made by utilizing the electrode resistances R1 and R2 of the discharge electrode Z from the connection points Y1 and Y2 to the electric discharge generation position S. Therefore, if the discharge electrode Z is made of copper or like low resistivity material, the sensitivity of detection is reduced, and in the case of the copper electrode, the electric discharge generation position detection is substantially impossible. FIG. 12(B) shows relation in case of using a copper electrode. In this case, the standardized value is not changed virtually with the electric discharge generation position.